Analyzing the stability of the power system by using a few machines is promising for transient stability assessment. A hybrid direct-time-domain method that is fully based on the thinking of partial energy function is proposed in this paper. During post-fault period, a pair of machines with high rotor speed difference is defined as couple machines, and the stability analysis of the system is transformed into that of several pairs of couple machines. Based on the prediction of power-angle curve of couple machines within a sampling window after fault clearing, the proposed method avoids the definition of Center of Inertia (COI) and it can also evaluate the stability margin of the system by using the predicted power-angle curve. Simulation results demonstrate its effectiveness in transient stability assessment.